Hybridization as the Engine of Adaptive Radiation

Adaptive radiation is the process in which organisms rapidly diversify to fill empty ecological niches. A textbook example is the radiation of Darwin’s Finches on the Galapagos Islands. These birds diversified in beak morphology occupying different ecological feeding niches. A similar story unfolded on the Hawaiian islands where Honeycreepers evolved into endless forms most beautiful. And it will get even better when you add hybridization to the mix!

Several biologists have proposed that hybridization between evolutionary lineages can cause rapid diversification of ecological phenotypes, thereby facilitating adaptive radiations. This plausible hypothesis has not been tested theoretically. Sounds like a challenge for Kotaro Kagawa and Gaku Takimoto, who constructed a mathematical model to simulate how hybridization can impact adaptive radiations.

Their mathematical model is quite, well, mathematical. So, I will focus on the outcomes. If you are interested in the actual model, you can read all about it in Ecology Letters. One of the key parameters in these analyses is the creation of new phenotypic variation by hybridization. This phenomenon is known as transgressive segregation. See, for instance, the unique colors and shapes of hybrid orchids or the extreme size of lion x tiger crosses.

However, the amount of phenotypic variation introduced by hybridization has important consequences for the outcome of the model. Too much variation and the fitness of the hybrids decreases (i.e. they are too different from their parents), ultimately leading to the collapse of hybrid populations. But if the amount of phenotypic variation in hybrids increases fitness, a hybrid population might florish and give rise to a hybrid species. With regard to adaptive radiations, hybrids with extreme phenotypes might reach new ecological niches that are inaccesable to other species. In a sense, hybridization allows populations to jump over fitness valleys onto distant adaptive peaks. In other words, hopping hybrids.